Optical scanning equipment

ABSTRACT

An aerial infra-red line-scanning system is provided in which rotation of a scanning rotor not only scans the terrain below the aerial vehicle and reflects the scanning beam on to an infra-red detection cell but also traverses a plurality of recording beams widthwise in succession across a film record transported close to the rotor in a direction parallel to the rotor axis. The recording beams are modulated in accordance with the signal output of the IR cell. The film is constrained, in the region where it is scanned by the recording beams, to a widthwise curvature centered on the axis of the rotor.

United States Patent Gull 1 -1 Feb. 6, 1973 [54] OPTICAL SCANNING EQUIPMENT [56] References Cited [75] Inventor: Frank Arthur Gull, St. Albans, En- UNITED STATES A NT gland 3,287,559 11/1966 Barnes ..250/83.3 HP [73] Assignee: Hawker Siddeley Dynamics Limited, 3,509,345 4/1970 Astheimer ..250/65R HeftfQrdShife En land 2,895,049 7/1959 Astheimer et al ..250/65 R [22] Filed: 1971 Primary ExaminerJames W. Lawrence 2 APP] 204 4 3 Assistant Examiner-Harold A. Dixon Attorney-Arthur E. Dowel], Jr. et al. Related US. Application Data [63] Continuation-impart of Ser. No. 811,698, March 27, [57] ABSTRACT 1969. An aerial infra-red line-scanning system is provided in which rotation of a scanning rotor not only scans the terrain below the aerial vehicle and reflects the [30] Forelgn Apphcanon Pnomy Dam scanning beam on to an infra-red detection cell but April 1, 1968 Great Britain ..5,299/68 also traverses a plurality of recording beams widthwise in succession across a film record transported close to 52 U.S. Cl. ..250/s3.3 11, 250/65 R the rotor in a direction parallel to the rotor axis- The [51] Int. Cl ..G0lt 1/16 recording beams are modulated in accordance with 58 Field of Search ..250/65 T, 83.3 H, 83.3 HP, the Signal Output of the IR cell- The film is 250/65 R strained, in the region where it is scanned by the recording beams, to a widthwise curvature centered on the axis of the rotor.

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I SHEET 7 UF 8 OPTICAL SCANNING EQUIPMENT tion of the invention lies in photographing the landscape passing beneath an airborne or similar vehicle, particularly to obtain a thermal picture of the infra red part of the spectrum.

It is an object of the invention to achieve scanning equipment for this type of duty which is unusually compact and uses fewer components than systems available hitherto, and for which the driving power required is kept advantageously low.

For infra-red linescan systems which record directly on to film via recording optics that scan a light spot widthwise across the film in synchronization with the infra-red scanner, the angular across-track coverage is dependent on the length of arcv traversed by the spot, and hence also on the curvature of the film. For a given film curvature there is a minimum distance between the take-up and supply film spools which allows the film to be curved without tearing. It is this distance which to a large extent determines the size of the recording system.

In a modern airborne reconnaissance linescan system, which will typically scan an angle of 90 or more, the infra-red scanning and detection system is comparable in size with the recorder system. In order to reduce the overall size of the equipment to a minimum it is essential to closely integrate the two systems. The total size is then governed by the distance between film spools determined by the criterion given above. 1

According to the present invention, there is provided a scanning and recording system to be carried by a vehicle, comprising a scanning rotor mounted to rotate about an axis parallel to the direction of motion ofsaid vehicle and having a plurality of reflecting faces, detection means receiving beams reflected from said plurality of reflecting faces and giving a signal output that varies in accordance with beam strength falling thereon,

. record medium transport means transporting a record therewith and projecting at least one recording beam radially with respect to said axis of rotation which recording beam passes repeatedly widthwise across said record medium band transported close to said rotor as said rotor rotates, means modulating said recording. beam in accordance with variations in the signal output of said detection means, and means constraining said record medium band, in the region of its travel where his scanned by said recording medium beam, to a widthwise curvature centered on said axis of rotation.

Preferably, for optical scanning, the rotor is of square cross-section with four mirror faces. In the infra red case the detection means will normally be an infra red sensitive electrical cell. All the picture information can be detected by a sensitive surface or surfaces of a single cell if the beams from the rotor faces are focused by collector mirrors having mirror faces which are parts of a paraboloid and an optical element such as a ridge mirror is employed to reflect on to the detector cell beams approaching it on either side from the two collector mirrors.

The arrangement gives space immediately above or alongside the scanning rotor (assuming the field of scan is below the scanner) for further equipment in the system and especially for transport of the record medium. To produce a photographic record in the infra red case the signal output of the detector cell may be employed to modulate a source of white light projected as a focused recording beam for writing on ordinary photographic film.

The same rotor is employed not only to perform the line scan in the viewing optical system but also to provide the line scan for producing the photographic record. If the rotating optical system is a multiple arrangement projecting the recording beam along a number, say four, of equiangularly-spaced radial paths each radial beam or light pencil will pass in succession across the film so writing successive lines thereon as the film travels. By situating the modulated source of white light, e.g. a glow lamp, on the axis of the scanning rotor, at or near one end of the rotor, an exceptionally compact assembly is achieved wherein all the recording optics are carried in or around the rotor.

By way of example, several embodiments of the invention will now be described with reference to the accompanying drawings, which show equipment to be housed in an airborne vehicle, i.e., a drone, for taking an infra red photographic record of the terrain below. In the drawings:

FIG. 1 is an exploded pictorial view of one embodiment,

FIG. 2 is a diagram of the scanning optics seen along the axis of the rotor of the embodiment of FIG. 1,

FIG. 3 is a pictorial diagram illustrating the operation of the scanning optics of the embodiment of FIG. 1,

FIG. 4 is a diagram of a film recorder seen in the same direction as that in FIG. 2,

FIG. 5 shows the film recorder in the direction of the arrow 5 of FIG. 4,

FIG. 6 is a diagram of the recorder optics seen from the same viewpoint as that in FIG. 5,

FIG. 7 shows the recorder optics on the line 7-7 of FIG. 6, I

FIG. 8 shows diagrammatically an alternative optical system seen looking along the rotor axis,

FIG. 9 is a diagram of another embodiment seen at right angles to the rotor axis,

FIG. 10 is a diagram of yet a further embodiment seen at right angles to the rotor axis, and

FIG. 11 is a view of the embodiment of FIG. 10 along rotor axis.

Referring firstly to FIGS. 1 to 7, the equipment is designed as a self-contained unit to fit into a drone camera compartment 10 which takes the form generally of a horizontal cylinder. The line-scan device consists of a main frame 13 of light alloy supporting two split parabaloid collector mirrors ll, 12 disposed at either side of an aperture 14 cut into'the lower half of the drone skin and structure. Between the two mirrors ll, 12 spins an optically polished square scanner l5, rotating at 7,500 rev/min. about a horizontal fore and aft axis to scan the field of view across the terrain beneath the drone through the aperture 14.

Immediately above and attached to the scanner mounting 16 is a film transport holder 17 and film drive mechanism. Above this, centrally mounted on top of the main frame 13 is the infra red detector 18, electronics pack 19, and a DC motor 20 that drives the scanner rotor through a train of gears 21 at the forward end of the structure. The film transport comprises a supply compartment 22 for an unused film cassette at the front end of a compartment 23 for an exposed film cassette at the rear, the film 24 (70mm) travelling substantially horizontally from front to rear between the two. Quick access to the used film cassette is obtained by removing the scanner mounting 16 and film transport 17 in one piece, by loosening four quick-release bolts 25, leaving the remainder of the line scan in situ, and opening a hatch 26 in an end plate 27 of the compartment 23. The choice of the four-sided mirror results from the fact that this configuration gives a reasonably optimum collecting area, consistent with a modest power to drive the rotor and furthermore enables a reasonably constant aperture'to be presented during the effective scan.

From this arrangement, the largest collecting aperture can be obtained for the drone space and power available. Since power absorbed in windage losses is approximately proportional to the fifth power of the rotor diameter and directly proportional to length it is important to obtain the desired aperture area using a rotor which can be long and have 'a small diameter, rather than vice versa. The scanner should therefore be designed, firstly, to occupy the maximum length of aperture available in the drone skin and then to have the largest possible diameter within the limit set by the power available to drive it. A system with a square mi'rror can operate at atmospheric pressure, without the need for a sealed window to enable a lower pressure to be used to reduce windage.

The fixed optical components collect the radiation reflected from the scanning rotor and focus it on to the infra red detector. These surfaces are all reflecting to avoid transmission losses which would occur if lenses were used. The split paraboloid collector mirrors l1, 12 are used to gather radiation from each side of the rotor 15 and to focus it on to the detector 18 via a small reflecting ridge mirror 28. The split collectors allow the optical system to have minimum height, and provide 7 space between and above them for the photographic recording equipment. As the materials most transmissive of infra red radiation are opaque to visible radiation reflextive optics have the further advantage that optical alignment can be more easily performed. This is because:

a. materials reflective to [.R. radiation have reasonably high reflectivity at visible wavelengths,

b. the optical power is independent of wavelength. Visualv inspection canthus be used instead of electrooptic conversion of LR. to visible radiation or to an electrical signal proportional to focused intensity and the visible image is identical with the image which will be produced with'LR. radiation except to diffraction limiting'effects and scattering from surface imperfections which are both wavelength dependent.

The paraboloid surfaces of the collector mirrors give a resolution betterthan 1 milliradian about the scanner axis. These mirrors-can'be made in glass from a single paraboloid and coated with a special gold-contain reflecting surface, coating to provide durability and high reflecting properties out to 14 microns wavelength. Such surfaces will not be susceptible to deterioration with time and a salt laden atmosphere, and can be cleaned by hand without fear of damage. Instead of glass, a suitable compression moulded plastic, or a thin plastic coating on an aluminum or honeycomb rigid substrate, may be employed giving savings in production cost and in weight of the system.

The optical system is auto-collimating with the scanner rotor removed so that it can be focused without the need for an external collimated light source.

The varying signal produced by the infra red detector 18, due to the incoming radiation falling thereon, is used through the medium of a converter unit to modulate white light which then writes upon the photographic film. For this purpose, the square scanner rotor 15 is hollow with an enlargement mid-way along its length constituting a housing 29 for recorder optics. lnto one end of the hollow rotor there projects a stationary glow tube 30 the light output of which is modulated in accordance with the signal output of the infra red detector 28. This light is foc'usedinto a beam by a lens 31 and passes axially through the rotor to be reflected back at the far end by a mirror 32.

The rotating housing 29 contains a central assembly of mirrors or prisms 33 that reflect the return beam from the mirror 32 out radially along four paths spaced at angles. ln each of these radial paths there is a lens system 34 for focusing the respective beam on to the film 24 above the scanner rotor. The four beams so focused traverse the width of the film in succession as the scanner rotates and thereby write" four successive lines of the film record. To maintain correct focus, the film is constrained by a curved guide so that, in the region of its travel where the lines of the record are written on it, it is curved widthwise about the axis of the scanner rotor. The speed of the scanner rotor is constant while the film transport speed can be variedto suit the speed of the vehicle.

The whole of the detecting and recording assembly thus forms aparticularly compact assembly.

Referring now to FIG. 8, this shows an alternative optical system. The square scanning rotor 15 is, in this case, flanked by two focusing collector mirrors 40, 41 which reflect energy collected from the scanner faces on to two IR detector cells 42, 43, respectively, which detector cells are situated between the mirrors and the scanning rotor approximately on the same horizontal level as the rotor axis. As the system scans, the recovered IR energy varies from a maximum on the cell 43 and a minimum on the cell 42 to a minimum on cell 43 and a maximum on the cell 42. At the vertical position of the scanning beam the energy is equally divided between the two detector cells. To ensure a constant IR conversion gain at all across-track angles the outputs of cells 42 and 43 are added, after amplification by amplifiers 44, 45, and a balance potentiometer 46 is preset to compensate for production differences in detector efficiency and amplifier gain.

The mirrors 40 and 41 are sections of a parabola of sufficient length to collect energy from the full length of the rotor which will have a lengthto width ratio of typically 4 l.

FIG. 9 shows diagrammatically another embodiment. The scanner in this system is a double-sided plane mirror 47, rotating about a horizontal axis 48 at 45 to the mirror face. Radiation reflected from the mirror 47 is collected by parabolic mirror segments 49, 50 disposed on the axis 48 at opposite sides of the mirror 47 and is focused via plane mirrors 51, 52 on to detector cells 53, 54. The video signal from each detector cell is used to modulate the output of a light source 55 or 56, disposed at the opposite side of the mirror 47, the light from which is reflected by a mirror surface 57 on the IR scanning mirror 47 to a microscope objective 58. The image from this objective is scanned across the moving film 24, constrained in an are about the scanner rotational axis as in FIG. 7. The scanning mirror 47 is made double-sided, and the system symmetrical, in order to give two scans per revolution. The detector cells and light sources are housed in a tube 59 along the scanner axis 48 such that the remainder of the system rotates about them.

FIGS. and 11 show a system using a four-sided pyramid mirror scanner 60. This scans the radiation received by a fixed spherical collector mirror 61, disposed above the rotor, via four rotating infra red transfer lenses 62, one corresponding to each face of the scanner mirror 60. A circular field stop 63 is disposed about the scanner axis 64 to control the selection of the area of mirror 61 from which radiation is reflected onto the detector cell 65 so that no unique axis exists. The possibility of extra-axial aberrations is thereby avoided. The radiation from the faces of the scanner 60 is directed to a detector cell 65, the video signal from which is used to modulate a light source 66 disposed on the scanner axis. Light from the source 66 is directed via a second rotary four-sided pyramid mirror 67, mounted back to back with the scanner 60, to each of four microscope objectives 68 positioned adjacent to the infra red lenses 62. The film record transport 69 is positioned to one side of the scanner and the. film 24 is exposed, over an are equal to the are scanned by the detector, to the beam from each objective in turn. I

5 In all these systemsthe detector cell is not limited to a single infra red-sensitive element but can be multielement, the recorders then having a corresponding number of individually modulated light sources.

What I claim is:

l. A scanning and recording system to be carried by a vehicle, comprising a scanning rotor mounted to rotate about an axis parallel to the direction of motion of said vehicle and having a plurality of reflecting faces, detection means receiving beams reflected from said plurality of reflecting faces and giving a signal output that varies in accordance with beam strength falling thereon, record medium transport means transporting a record medium band along a path extending parallel to said axis of rotation and close to said rotor, recording beam projection means mounted on said rotorto rotate therewith and projecting at least one recording beam radially with respect to said axis of rotation which recording beam passes repeatedly widthwise across said record medium band transported close to said rotor as said rotor rotates, means modulating said recording beam in accordance with variations in the signal output of said detection means, and means constraining said record medium band, in the region of its travel where it is scanned by said recording beam, to a widthwise curvature centered on said axis of rotation.

2. A system according to claim 1, wherein the scanning is optical scanning and said detection means is at least one infra red sensitive cell.

3. A system according to claim 1, wherein said rotor is generally of square cross-section with four equiangularly-disposed mirror faces.

4. A system according to claim 1, wherein the rotor comprises a double-sided plane mirror set in a plane at 45 to the axis of rotation, and focusing mirrors disposed at opposite sides of said plane mirror and centered on the rotor axis to collect and focus the beams from said plane mirror on to said detection means.

5. A system according to claim 1, wherein the rotor comprises a four-sided pyramidal'mirror which reflects on to said detection means the radiation from a fixed spherical collector mirror.

6. A system according to claim 1, disposed in an aerial vehicle for scanning the terrestrial scene below and mounted in an open compartment at ambient atmospheric pressure.

7. A system according to claim 1, wherein said recording beam projection means comprises multiple optical focusing systems projecting said recording beam along a plurality of equiangularly-spaced radial paths, whereby plural radial beams pass in succession across said record medium band so recording successive lines thereon as said record medium band travels.-

8. A system according to claim 7, wherein a light source modulated in accordance with said detector means signal output is disposed on the axis of said rotor, and said recording beam projection beams further comprises central reflecting optical means within said rotor, the light from said modulated light source passing axially within the rotor and being reflected to said multiple optical systems by said central reflecting optical means.

9. A system according to claim 1, further comprising twin fixed focusing reflectors situated one on either side of said rotor, to reflect beams from the faces thereof on to said detection means.

10. A system according to claim 9, wherein said fixed focusing reflectors are mirrors with their reflecting faces forming parts of a paraboloid and disposed to reflect beams from said rotor faces upward in two mutually converging beams, and a ridge mirror is situated and oriented to reflect on to said detection means beams approaching it on either side from said two fixed paraboloid mirrors.

11. A system according to claim 10, wherein said record medium band is transported between said mutually converging beans, and between said detection means and said rotor. 

1. A scanning and recording system to be carried by a vehicle, comprising a scanning rotor mounted to rotate about an axis parallel to the direction of motion of said vehicle and having a plurality of reflecting faces, detection means receiving beams reflected from said plurality of reflecting faces and giving a signal output that varies in accordance with beam strength falling thereon, record medium transport means transporting a record medium band along a path extending parallel to said axis of rotation and close to said rotor, recording beam projection means mounted on said rotor to rotate therewith and projecting at least one recording beam radially with respect to said axis of rotation which recording beam passes repeatedly widthwise across said record medium band transported close to said rotor as said rotor rotates, means modulating said recording beam in accordance with variations in the signal output of said detection means, and means constraining said record medium band, in the region of its travel where it is scanned by said recording beam, to a widthwise curvature centered on said axis of rotation.
 1. A scanning and recording system to be carried by a vehicle, comprising a scanning rotor mounted to rotate about an axis parallel to the direction of motion of said vehicle and having a plurality of reflecting faces, detection means receiving beams reflected from said plurality of reflecting faces and giving a signal output that varies in accordance with beam strength falling thereon, record medium transport means transporting a record medium band along a path extending parallel to said axis of rotation and close to said rotor, recording beam projection means mounted on said rotor to rotate therewith and projecting at least one recording beam radially with respect to said axis of rotation which recording beam passes repeatedly widthwise across said record medium band transported close to said rotor as said rotor rotates, means modulating said recording beam in accordance with variations in the signal output of said detection means, and means constraining said record medium band, in the region of its travel where it is scanned by said recording beam, to a widthwise curvature centered on said axis of rotation.
 2. A system according to claim 1, wherein the scanning is optical scanning and said detection means is at least one infra red sensitive cell.
 3. A system according to claim 1, wherein said rotor is generally of square cross-section with four equiangularly-disposed mirror faces.
 4. A system according to claim 1, wherein the rotor comprises a double-sided plane mirror set in a plane at 45* to the axis of rotation, and focusing mirrors disposed at opposite sides of said plane mirror and centered on the rotor axis to collect and focus the beams from said plane mirror on to said detection means.
 5. A system according to claim 1, wherein the rotor comprises a four-sided pyramidal mirror which reflects on to said detection means the radiation from a fixed spherical collector mirror.
 6. A system according to claim 1, disposed in an aerial vehicle for scanning the terrestrial scene below and mounted in an open compartment at ambient atmospheric pressure.
 7. A system according to claim 1, wherein said recording beam projection means comprises multiple optical focusing systems projecting said recording beam along a plurality of equiangularly-spaced radial paths, whereby plural radial beams pass in succession across said record medium band so recording successive lines thereon as said record medium band travels.
 8. A system according to claim 7, wherein a light source modulated in accordance with said detector means signal output is disposed on the axis of said rotor, and said recording beam projection beams further comprises central reflecting optical means within said rotor, the light from said modulated light source passing axially within the rotor and being reflected to said multiple optical systems by said central reflecting optical means.
 9. A system according to claim 1, further comprIsing twin fixed focusing reflectors situated one on either side of said rotor, to reflect beams from the faces thereof on to said detection means.
 10. A system according to claim 9, wherein said fixed focusing reflectors are mirrors with their reflecting faces forming parts of a paraboloid and disposed to reflect beams from said rotor faces upward in two mutually converging beams, and a ridge mirror is situated and oriented to reflect on to said detection means beams approaching it on either side from said two fixed paraboloid mirrors. 